The present invention relates to a method of separating air by cryogenic distillation to produce a gaseous product at a high delivery pressure. More particularly, the present invention relates to such an apparatus and method in which the product is oxygen and in which the oxygen as a liquid is pumped to the high delivery pressure prior to its vaporization and delivery.
Air is separated into its components, for instance oxygen, nitrogen, argon and etc., at low temperatures within one or more distillation columns. Typically, after filtering of the air to remove particulate material, the air is compressed, the heat of compression is removed, and the air is then purified. Purification is commonly effectuated by an adsorbent system to remove carbon dioxide and hydrocarbons. Thereafter, the air is cooled to at or near its dewpoint in a main heat exchanger and is introduced into a distillation column. The distillation column contains trays or packing to contact descending liquid and ascending vapor phases of the air. As a result of such contact, the ascending vapor becomes ever more concentrated in the more volatile components of the air, for instance nitrogen, and the descending liquid phase becomes ever more concentrated in the less volatile components of the air, for instance, oxygen.
Two columns, connected in a heat transfer relationship, are used to produce both higher purity liquid oxygen and nitrogen products. Although there is some demand for the nitrogen product to be delivered at pressure, it is more commonly required that the oxygen product be delivered at high pressure. In order to deliver an oxygen product at high pressure, liquid oxygen from a lower pressure column is pumped to the delivery pressure and is then vaporized against a portion of the incoming air to produce a gaseous product at pressure which is discharged at ambient temperature from the warm side of the main heat exchanger. Very often, a thermosyphon type heat exchanger is interposed between the main heat exchanger and the lower pressure column in order to effectuate vaporization of the liquid oxygen.
In order for the liquid oxygen to be vaporized by the air, the portion of the air that is used for this purpose is sufficiently compressed by a booster compressor that a temperature difference will be maintained between the air and the liquid oxygen product to be vaporized. The head of the oxygen to be vaporized within a thermosyphon reboiler is, however, not constant. As result, the boiling temperature of the liquid oxygen increases to a maximum at an intermediate location of the boiler. In order to maintain the requisite temperature difference throughout the thermosyphon reboiler, the pressure of the incoming air must be sufficiently boosted to maintain a temperature difference between the air and the maximum temperature.
As will be discussed, the present invention provides an apparatus and method in which linear temperature difference characteristics are preserved between the incoming air and liquid oxygen product to be vaporized so that a minimum temperature difference between the air and the oxygen can be maintained. This in turn, allows less compression than a prior art apparatus and method and therefore, a concomitant savings in energy.